RF circuitry is prone to producing communication errors from noise and/or transients in the RF power supply. As a result, the power supplies used in mobile wireless communications devices must be able to provide a steady supply voltage with minimal noise.
Due to their inherent low noise characteristics, RF sections in mobile wireless communications devices are typically powered by a linear voltage regulator. However, linear voltage regulators are typically very inefficient. When the mobile communication device is in a sleep or low power consumption mode, the inefficiency of the linear voltage regulator is of minimal concern. However, when the RF section of the mobile communication device is active, the inefficiency of the linear voltage regulator can dramatically shorten battery life of the device.
In contrast to linear power supplies, switch-mode power supplies (SMPSs) or push-pull voltage regulators, are typically at least 90% efficient. However, due to the noise generated from their switching transistors, SMPSs are generally not desirable for use with RF sections. Therefore, attempts have been made to devise a more efficient low-noise power supply suitable for use with the RF section of a mobile wireless communication device.
For instance, Nokkonen (U.S. Pat. No. 6,441,591) describes a power supply that includes a battery, a SMPS and a linear voltage regulator each coupled to the battery, and a SMPS controller connected to the SMPS. The SMPS controller is configured with two operational modes. In the first mode (activated when the load current is high), the SMPS is active, and the linear voltage regulator is powered by the SMPS. In the second mode (activated when the load current is low), the SMPS is inactive, and the linear voltage regulator is powered by the battery.
Since the efficiency of a linear voltage regulator diminishes with the voltage drop across the regulator, the voltage applied to the input of the linear regulator during the first (high current mode) is greater than the voltage applied during the second (low current mode). Therefore, the efficiency of the power supply is limited by the ability of the SMPS controller to select the appropriate operational mode based on the prevailing load current.